“The Barter skill affects the prices you get for buying and selling items. In general, the higher your Barter skill, the lower your prices on purchased items.”

Like it is said in the manual, you gain an advantage on trade prices, which is fairly important. The more money you get for your loot, the more ammo you can buy. The more ammo you have, the easier your life will be. Being able to get your hands on more ammo, armor, and meds will make the game easier, obviously. There is, however, plenty of loot in the game, so while this skill helps, it’s not a skill you should lose sleep over.